More than 350 people a year are killed when a car strikes the back of a big truck and slides underneath. There are safety standards to prevent these so-called truck underride accidents, but a new study shows the protections aren't working.

Rear impact guards, fastened to the backs of big rigs, are designed to stop cars and prevent them from sliding underneath. The Insurance Institute for Highway Safety (IIHS) put them to the test. The Institute crashed a 2010 Chevy Malibu, traveling 35 miles an hour, into the back of parked trailers. The rear guard that meets the U.S. standard gave way, and the car slid right under the trailer, crushing the vehicle. If there had been real occupants instead of crash dummies in the front seat, the IIHS said they would not have survived.

"Our tests show how easily some of these guards are failing at relatively moderate speeds," said institute president Adrian Lund. "The standards need to be stronger. These crashes don't have to be deaths or serious injuries."

Canada requires rear impact barriers that are 75 percent stronger than those in the U.S. In the IIHS crash tests, the Canadian-style guard held up properly when the car hit it.

For Nancy Meuleners, a rear under-ride crash has meant 40 surgeries and a changed life -- she lost her jaw and parts of her tongue.

"Speaking can be an issue. Eating. I can't eat normal foods," Meulener said.

Meuleners, of Bloomington Minnesota, has lobbied to get stronger rear guards, "We need lower, safer, more energy-absorbing guards," she said. She is understandably nervous when driving near a big rig. "They are a danger to me and to the American public, I feel, without proper underride bars on them."

"It doesn't provide the kind of underguard protection that clearly is called for," said Bill Graves, president of the American Trucking Associations, after being shown the test video.

Big Rig Threat: Cars Slide Under Trailers

Graves said, though, that there the right barrier design is a "complicated puzzle to solve."

"That's the question the federal government has been wrestling now for many years, is what's the strength we want," he said." What's too much? And what's not enough?"